1. Field of the Invention
This invention relates to an electron beam system and, more particularly, to electron beam system uniquely suited for writing both small and large areas, preferably, non-repetitive patterns, such as is required in the micro-fabrication of large scale integrated circuit patterns.
2. Description of the Prior Art
Electron beam columns have previously been used in systems for the micro-fabrication of large scale integrated circuits. Such columns have utility in writing patterns on substrates, such as on semiconductor wafers covered with photoresist, which is developed to form openings to the semiconductor wafer for further processing in integrated circuit fabrication. They also have utility in making masks for optical exposure, magnetic bubble fabrication, and optical data storage media fabrication. The function of any such system is to write a given pattern over a given area with a specified charge density and with adequate edge resolution in the shortest possible time. These patterns are made up of a plurality of pattern elements. The electron beam column must be designed such to achieve maximum throughput of semiconductor wafers while maintaining high quality exposure of the patterns. In cases where compatibility with other lithography systems is required, absolute deflection accuracy and especially repeatability are essential. In addition, for multi-level patterns, it is essential to be able to overlay the successive layers of the pattern with a high degree of accuracy. In view of the above requirements, various electron beam lithography systems have been designed and utilized. A typical electron beam system utilized in connection with integrated circuit micro-fabrication may include an electron beam source, condenser lenses, alignment stages, demagnification lens stages, a projection lens, a deflection unit, and a target area. For example, U.S. Pat. No. 3,644,700 issued Feb. 22, 1972 to Kruppa et al, describes a typical electron beam column. Other electron beam columns and components therefor are described in U.S. Pat. No. 3,949,228 to Ryan and U.S. Pat. No. 3,984,678 to Loeffler. Another type of electron beam column which has been adapted for micro-fabrication of integrated circuits utilizes a fixed shape beam spot. An example of such a electron beam column is described in the Kruppa patent. A square beam column exposes a number of image points in parallel and gains an equivalent factor in throughput over Gaussin spot or round beam electron beam columns. With a square beam column, the electron beam is focussed to provide a demagnified image of an aperture called the beam shaping aperture, which is square. The size of the focussed square spot is generally chosen to be the same as the minimum pattern line that is required and the optical system is designed so that the edge resolution of the spot is considerably less than this. Each pattern element is written by moving the shaped beam in discrete jumps so that the pattern is written as a series of squares.
The square beam imaging system has advantages over the Gaussin round beam system, as enumerated in the publication "New Imaging and Deflection Concepts For Performing Micro-fabrication Systems", by H. C. Pfeiffer, Journal of Vacuum Science and Technology, December 1975, Vol. 12, No. 6, pages 1170-1173. A further enhancement of the square beam imaging system is a variable shape electron beam column as described in copending application Ser. No. 771,235, now abandoned, for Method and Apparatus of Forming a Variable Sized Electron Beam, H. C. Pfeiffer, P. M. Ryan and E. V. Weber, filed Feb. 23, 1977, and assigned to the assignee of the present application. In this variable shape beam system, the deflection unit is placed between two beam shaping apertures. With the deflection unit switched off, the second square aperture is fully illuminated, thereby resulting in a square focussed spot of maximum dimensions. When the deflection unit is energized, the illumination of any portion of the second square aperture can be blanked off thereby producing a rectangle or a square spot of any desired size, up to the maximum, containing the same current density as the original square spot.
The above-described electron beam systems utilize a raster scan to write the patterns. The advantage of the raster scan is that the writing is the same for all of the patterns and errors may be measured and corrected prior to writing by the technique described in U.S. Pat. No. 3,644,700. The disadvantage of the raster scan is that the total field area must be covered even though the written area is only a small percentage of the field.
In contrast to the raster scan electron beam system, there is a system known as vector scan in which only the written area is scanned with a beam while non-pattern areas are skipped. This means that the exposure time can be reduced by the percentage of unwritten area. In the vector scan mode of computer controlled circuit pattern writing, as described in "A Computer-Controlled Electron-Beam Machine For Microcircuit Fabrication", by T. H. P. Chang and B. A. Waldman, in IEEE Transactions on Electron devices, May 1972 at pages 629-635, the pattern is divided into a series of rectangles and parallelograms. The electron beam is then controlled by the computer to access the rectangles and parallelograms sequentially to expose the inside of them. Throughput of such a system is dependent on the speed at which the semiconductor wafer can be exposed by the electron beam. Since exposure speed is limited by the current delivered to the wafer surface by the beam and the beam current is determined by the beam size, one of the major drawbacks of such a system is the limitation on throughput necessitated by the small beam size required to insure edge sharpness and accuracy. Prior art systems have attempted to solve this problem by making trade-offs between beam size on the one hand and edge definition and accuracy on the other. It was not feasible in the prior art systems to enlarge the size of the electron beam used to fill the inside of the circuit pattern, because the machine had to be powered down to change lenses in order to change the beam size. Therefore, the prior art electron beam systems were restricted to a speed determined by the electron beam size required for proper edge definition and accuracy. In addition, the vector writing approach with electron beam is inherently less accurate than a raster scan which has been corrected by a calibration scheme independent of the written patterns.
The above described prior art electron beam systems employ only a single deflection means, usually electric. An article by 0. C. Woodard et al entitled, "Variable Spot Shape Control Electronics", J. Vac. Sci Technol 16(6) November/December 1979 describes a dual deflection electron beam system for raster writing semiconductor wafers. The dual deflection aspect of this system gives greater speed and accuracy than a single deflection means but, because of the raster scan, the total field still must be covered to write only a small portion of the field.